The term `microporous` is used herein to identify porous or cellular materials in which the ultimate size of the cells or voids is less than the mean free path of an air molecule at NTP, i.e. of the order of 100 nm or smaller. A material which is microporous in this sense will exhibit very low transfer of heat by air conduction (that is collisions between air molecules). Such microporous materials include aerogel, which is a gel in which the liquid phase has been replaced by a gaseous phase in such a way as to avoid the shrinkage which would occur if the gel were dried directly from a liquid. A substantially identical structure can be obtained by controlled precipitation from solution, the temperature and pH being controlled during precipitation to obtain an open lattice precipitate. Other equivalent open lattice structures include pyrogenic (fumed) and electro-thermal types in which a substantial proportion of the particles have an ultimate particle size less than 100 nm. Any of these particulate materials, based for example on silica, alumina or other metal oxides, may be used to prepare a composition which is microporous as defined above.
The microporous insulation typically comprises a dry particulate microporous material as defined hereinabove mixed with ceramic fibre reinforcement, titanium dioxide opacifier and, for high-temperature use, a small quantity of alumina powder to resist shrinkage. Such microporous insulation material is described in GB-A-1 580 909.
When a heating element in the form of an elongate strip of electrically conductive material is partially embedded edgewise into a base of insulation material, such as microporous thermal and electrical insulation material, to form a radiant heater, during operation of the heater the strip reaches a high temperature required for satisfactory operation of the heater. However, the entire strip comprising exposed and embedded regions operates at a high temperature and it would be advantageous if the embedded portion of the strip could be arranged to attain a lower temperature than the exposed region when the heater is operated.
It is known from GB-A-1 569 588 to provide a heating conductor strip which is slotted alternately from opposite edges and is provided with spaced anchoring tabs which extend from the strip and penetrate an underlying insulating sheet. As acknowledged in GB-A-1 569 588 the slots have the disadvantageous effect of reducing the rigidity of the heating conductor strip in all directions. Moreover, the anchoring tabs are spaced at a distance of several slots from one another. The overall effect of the low rigidity heating conductor and the relatively wide spacing of the anchoring tabs gives rise to undesirable distortion of the heating conductor as a result of the regular cycles of heating and cooling to which the heating conductor is subjected. This is turn can lead to adjacent turns of the heating conductor becoming too close to each other and giving rise to an electrical short circuit or to the heating conductor coming into contact with a temperature sensor which is conventionally provided in radiant heaters for glass-ceramic smooth top cookers.
U.S. Pat. No. 4,292,504 describes an electric resistance heating unit in which the heating element comprises a thin, foil-like strip of expanded metal supported on edge substantially along its entire length on a board of insulating material. The heating element is either cemented or a close fit within a groove formed in the board. Such an expanded metal heating element occupies an undesirable amount of space within the heater and is relatively fragile: it additionally gives rise to the same flow of electric current in the portion of the heating element within the groove as in the exposed portion thereof and the embedded portion of the heating element therefore rises to an undesirably high temperature.
US-A-600 057 describes an electric heater in which a conductor stamped into one of a number of shapes is attached edgewise to a support by one or more layers of enamel. The conductor is shaped in order to prevent damage to the enamel which is used to attach the conductor to the support. By way of example, the conductor may be provided with a series of lateral projections, which may be of any desired shape, which projections are partially or entirely embedded in the enamel, with the body of the conductor being exposed.